text: Snf3 is homologous to multiple sugar transporters, it shares high similarity to the glucose transporters of rat brain cells and human HepG2 hepatoma cells, as well as to the arabinose and xylose transporters (AraE and XylE) of Escherichia coli. Based on this homology and on genetic studies, Snf3 was initially thought to be a high affinity glucose transporter.
  Later, it was found that Snf3 is not a glucose transporter, but rather a high affinity glucose sensor. It senses glucose at low concentrations and regulates transcription of the HXT genes, which encode for glucose transporters. If glucose is absent Snf3 is quiescent and transcription of the HXT genes is inhibited by a repressing complex. The complex consisting of several subunits such as Rgt1, Mth1/Std1, Cyc8 and Tup1 binds to the promoters of the HXT genes, thereby blocking their transcription.
  Snf3 is able to bind even low amounts of glucose due to its high affinity. The induction of Snf3 by glucose leads to the activation of YckI, a yeast casein kinase. This is followed by the recruitment of Mth1 and Std1 to the C-terminus of Snf3 which facilitates the phosphorylation of the two proteins by YckI.  Phosphorylated Mth1 and Std1 are subsequently tagged for proteasome dependent degradation by SCF, an E3 ubiquitin ligase. Therefore, the inhibitory complex misses two of its key components and cannot be assembled. Thus, repression of the HXT genes is abolished, leading to the expression of the glucose transporters and subsequently  glucose import.